Ambrox® is the trade name of the enantiomerically pure compound (−)-ambrox (3aR,5aS,9aS,9bR)-3a,6,6,9a-tetramethyldodecahydronaphtho[2,1-b]furan), a sought-after fragrance. Naturally occurring (−)-ambrox is the olfactory most important ingredient of ambergris, a digestion product of sperm whales.
The different diastereomers of (−)-ambrox have a similar scent, but sometimes differ in their odor thresholds (G. Ohloff, W. Giersch, W. Pickenhagen, A. Furrer, B. Frei, Helv. Chim. Acta 68 (1985) 2022. G. Frater, J. A. Bajgrowicz, P. Kraft, Tetrahedron 54 (1998) 7633). The odor threshold of 3a-epi-(−)-ambrox is higher than that of (−)-ambrox by a factor of 100. By contrast, 9b-epi-(−)-mbrox has half as high an odor threshold as (−)-ambrox for virtually the same odor quality. (+)-Ambrox is eight times weaker than the natural enantiomer. The racemate has an odor threshold of 0.5 ppb and barely differs from that of (−)-Ambrox® in its tonalities. (B. Schäfer Chemie in unserer Zeit 2011, 45, 374).
Homofarnesol is an important intermediate of synthesis processes for preparing Ambrox® (R. L. Snowden, Chemistry & Biodiversity 2008, 5, 958. J. and D. Leffingwell Specialty Chemical Magazine 2011, 30).
In particular, the cyclization of all E-homofarnesol of the formula Ia
produces diastereomerically pure or enantiomerically pure ambrox (Super acids: P. F. Vlad et al. Khimiya Geterotsiklicheskikh Soedinenii, Engl. Transl. 1991, 746; R. L. Snowden, Chemistry & Biodiversity 2008, 5, 958. Lewis acid-Brönsted acid: K. Ishihara et al. J. Am. Chem. Soc. 2002, 124, 3647. Mechanistic investigations: R. L. Snowden et al. J. Org. Chem. 1992, 57, 955.).
The literature describes various processes for preparing all E-homofarnesol:
(1) stereoisomerically pure (3E,7E)-homofarnesol can be prepared from (E,E)-farnesol via (E,E)-farnesal, C1 extension according to Wittig with methylenetriphenyl-phosphorane and subsequent terminal hydroboration of the conjugated diene in accordance with a synthesis described in the literature (D. S. Dodd et al. J. Org. Chem. 1992, 57, 2794).
However, this synthesis is not a technically-economically sensible route to (E,E)-homofarnesol. A technical process for preparing isomerically pure farnesol is not given.
(2) An alternative known in the literature for the synthesis of (3E,7E)-homofarnesol consists in the following procedure (A. F. Barrero et al. J. Org. Chem. 1996, 61, 2215.): a) distillative separation of (E/Z)-nerolidol, b) reaction of (E)-nerolidol with dimethylformamide dimethylacetal (DMFDMA) in a Büchi rearrangement to give the corresponding (3E/Z, 7E)-C16-amides, c) flash-chromatographic separation of the stereoisomeric amides and d) reduction of the (3E,7E)-amide to the corresponding (3E,7E)-homofarnesol with lithium triethylborohydride. Disadvantages of this route are the moderate yields and the required flash chromatography for separating the stereoisomers.

(3) A patent application from Henkel (WO 92/06063, Henkel Research Corporation) describes the carbonylation of (E)-nerolidol with the addition of catalytic amounts of the relatively expensive reagent palladium (II) chloride. Furthermore, the reaction takes place disadvantageously for the implementation at high CO pressures of ca. 70 bar.

(4) A further literature source (P. Kocienski et al. J. Org. Chem. 1989, 54, 1215.) describes the synthesis of homofarnesol from dihydrofuran. Each cycle requires the alkylation of 5-lithio-2,3-dihydrofuran with a homoallylic iodide followed by Ni(O)-catalyzed coupling with methylmagnesium bromide. The resulting homogeraniol can be converted to the corresponding iodide and the cycle is repeated. The synthesis is E-selective and produces the homofarnesol over 5 stages in an overall yield of ca. 70%.

The first three processes described above never arrive directly at the oxidation state of homofarnesol. Furthermore, expensive hydride reagents are required for reducing the homofarnesylic acid. Process (4) is economically unattractive on account of the reagents required.
The objective of the invention is therefore the provision of an improved process for the preparation of homofarnesol, in particular (3E,7E)-homofarnesol, and structurally analogous compounds.